We compare the structure of star-forming molecular clouds in different regions of Orion A to determine how the column density probability distribution function (N-PDF) varies with environmental ...conditions such as the fraction of young protostars. A correlation between the N-PDF slope and Class 0 protostar fraction has been previously observed in a low-mass star-formation region (Perseus); here we test whether a similar correlation is observed in a high-mass star-forming region. We used Herschel PACS and SPIRE cold dust emission observations to derive a column density map of Orion A. We used the Herschel Orion Protostar Survey catalog to accurately identify and classify the Orion A young stellar object content, including the cold and relatively short-lived Class 0 protostars (with a lifetime of ~0.14 Myr). We divided Orion A into eight independent regions of 0.25 square degrees (13.5 pc2); in each region we fit the N-PDF distribution with a power law, and we measured the fraction of Class 0 protostars. We used a maximum-likelihood method to measure the N-PDF power-law index without binning the column density data. We find that the Class 0 fraction is higher in regions with flatter column density distributions. We tested the effects of incompleteness, extinction-driven misclassification of Class 0 sources, resolution, and adopted pixel-scales. We show that these effects cannot account for the observed trend. Our observations demonstrate an association between the slope of the power-law N-PDF and the Class 0 fractions within Orion A. Various interpretations are discussed, including timescales based on the Class 0 protostar fraction assuming a constant star-formation rate. The observed relation suggests that the N-PDF can be related to an evolutionary state of the gas. If universal, such a relation permits evaluating the evolutionary state from the N-PDF power-law index at much greater distances than those accessible with protostar counts.
ABSTRACT The dust extinction curve is a critical component of many observational programs and an important diagnostic of the physics of the interstellar medium. Here we present new measurements of ...the dust extinction curve and its variation toward tens of thousands of stars, a hundred-fold larger sample than in existing detailed studies. We use data from the APOGEE spectroscopic survey in combination with ten-band photometry from Pan-STARRS1, the Two Micron All-Sky Survey, and Wide-field Infrared Survey Explorer. We find that the extinction curve in the optical through infrared is well characterized by a one-parameter family of curves described by R(V). The extinction curve is more uniform than suggested in past works, with ( R ( V ) ) = 0.18 , and with less than one percent of sight lines having R ( V ) > 4 . Our data and analysis have revealed two new aspects of Galactic extinction: first, we find significant, wide-area variations in R(V) throughout the Galactic plane. These variations are on scales much larger than individual molecular clouds, indicating that R(V) variations must trace much more than just grain growth in dense molecular environments. Indeed, we find no correlation between R(V) and dust column density up to E ( B − V ) 2 . Second, we discover a strong relationship between R(V) and the far-infrared dust emissivity.
Star formation in molecular clouds is intimately linked to their internal mass distribution. We present an unprecedentedly detailed analysis of the column density structure of a high-mass, ...filamentary molecular cloud, namely IRDC G11.11-0.12 (G11). We use two novel column density mapping techniques: high-resolution (FWHM = 2″, or ~0.035 pc) dust extinction mapping in near- and mid-infrared, and dust emission mapping with the Herschel satellite. These two completely independent techniques yield a strikingly good agreement, highlighting their complementarity and robustness. We first analyze the dense gas mass fraction and linear mass density of G11. We show that G11 has a top heavy mass distribution and has a linear mass density (Ml ~ 600 M⊙ pc-1) that greatly exceeds the critical value of a self-gravitating, non-turbulent cylinder. These properties make G11 analogous to the Orion A cloud, despite its low star-forming activity. This suggests that the amount of dense gas in molecular clouds is more closely connected to environmental parameters or global processes than to the star-forming efficiency of the cloud. We then examine hierarchical fragmentation in G11 over a wide range of size-scales and densities. We show that at scales 0.5 pc ≳ l ≳ 8 pc, the fragmentation of G11 is in agreement with that of a self-gravitating cylinder. At scales smaller than l ≲ 0.5 pc, the results agree better with spherical Jeans’ fragmentation. One possible explanation for the change in fragmentation characteristics is the size-scale-dependent collapse time-scale that results from the finite size of real molecular clouds: at scales l ≲ 0.5 pc, fragmentation becomes sufficiently rapid to be unaffected by global instabilities.
ABSTRACT We present key results from the Herschel Orion Protostar Survey: spectral energy distributions (SEDs) and model fits of 330 young stellar objects, predominantly protostars, in the Orion ...molecular clouds. This is the largest sample of protostars studied in a single, nearby star formation complex. With near-infrared photometry from 2MASS, mid- and far-infrared data from Spitzer and Herschel, and submillimeter photometry from APEX, our SEDs cover 1.2-870 m and sample the peak of the protostellar envelope emission at ∼100 m. Using mid-IR spectral indices and bolometric temperatures, we classify our sample into 92 Class 0 protostars, 125 Class I protostars, 102 flat-spectrum sources, and 11 Class II pre-main-sequence stars. We implement a simple protostellar model (including a disk in an infalling envelope with outflow cavities) to generate a grid of 30,400 model SEDs and use it to determine the best-fit model parameters for each protostar. We argue that far-IR data are essential for accurate constraints on protostellar envelope properties. We find that most protostars, and in particular the flat-spectrum sources, are well fit. The median envelope density and median inclination angle decrease from Class 0 to Class I to flat-spectrum protostars, despite the broad range in best-fit parameters in each of the three categories. We also discuss degeneracies in our model parameters. Our results confirm that the different protostellar classes generally correspond to an evolutionary sequence with a decreasing envelope infall rate, but the inclination angle also plays a role in the appearance, and thus interpretation, of the SEDs.
Abstract
Models aiming to explain the formation of massive black hole seeds, and in particular the direct collapse scenario, face substantial difficulties. These are rooted in rather ad hoc and ...fine-tuned initial conditions, such as the simultaneous requirements of extremely low metallicities and strong radiation backgrounds. Here, we explore a modification of such scenarios where a massive primordial star cluster is initially produced. Subsequent stellar collisions give rise to the formation of massive (104−105 M⊙) objects. Our calculations demonstrate that the interplay among stellar dynamics, gas accretion, and protostellar evolution is particularly relevant. Gas accretion on to the protostars enhances their radii, resulting in an enhanced collisional cross-section. We show that the fraction of collisions can increase from 0.1 to 1 per cent of the initial population to about 10 per cent when compared to gas-free models or models of protostellar clusters in the local Universe. We conclude that very massive objects can form in spite of initial fragmentation, making the first massive protostellar clusters viable candidate birth places for observed supermassive black holes.
We study the fragmentation of the nearest high line-mass filament, the integral shaped filament (ISF, line-mass ~400 M⊙ pc-1) in the Orion A molecular cloud. We have observed a 1.6 pc long section of ...the ISF with the Atacama Large Millimetre/submillimeter Array (ALMA) at 3 mm continuum emission, at a resolution of ~3″ (1200 AU). We identify from the region 43 dense cores with masses about a solar mass. 60% of the ALMA cores are protostellar and 40% are starless. The nearest neighbour separations of the cores do not show a preferred fragmentation scale; the frequency of short separations increases down to 1200 AU. We apply a two-point correlation analysis on the dense core separations and show that the ALMA cores are significantly grouped at separations below ~17 000 AU and strongly grouped below ~6000 AU. The protostellar and starless cores are grouped differently: only the starless cores group strongly below ~6000 AU. In addition, the spatial distribution of the cores indicates periodic grouping of the cores into groups of ~30 000 AU in size, separated by ~50 000 AU. The groups coincide with dust column density peaks detected by Herschel. These results show hierarchical, two-mode fragmentation in which the maternal filament periodically fragments into groups of dense cores. Critically, our results indicate that the fragmentation models for lower line-mass filaments (~16 M⊙ pc-1) fail to capture the observed properties of the ISF. We also find that the protostars identified with Spitzer and Herschel in the ISF are grouped at separations below ~17 000 AU. In contrast, young stars with disks do not show significant grouping. This suggests that the grouping of dense cores is partially retained over the protostar lifetime, but not over the lifetime of stars with disks. This is in agreement with a scenario where protostars are ejected from the maternal filament by the slingshot mechanism, a model recently proposed for the ISF. The separation distributions of the dense cores and protostars may also provide an evolutionary tracer of filament fragmentation.
We report Atacama Large Millimeter/submillimeter Array and Very Large Array continuum observations that potentially identify the four youngest protostars in the Orion Molecular Clouds taken as part ...of the Orion VANDAM program. These are distinguished by bright, extended, irregular emission at 0.87 and 8 mm and are optically thick at 0.87 mm. These structures are distinct from the disk or point-like morphologies seen toward the other Orion protostars. The 0.87 mm emission implies temperatures of 41-170 K, requiring internal heating. The bright 8 mm emission implies masses of 0.5-1.2 M assuming standard dust opacity models. One source has a Class 0 companion, while another exhibits substructure indicating a companion candidate. Three compact outflows are detected, two of which may be driven by companions, with dynamical times of ∼300 to ∼1400 yr. The slowest outflow may be driven by a first hydrostatic core. These protostars appear to trace an early phase when the centers of collapsing fragments become optically thick to their own radiation and compression raises the gas temperature. This phase is thought to accompany the formation of hydrostatic cores. A key question is whether these structures are evolving on freefall times of ∼100 yr, or whether they are evolving on Kelvin-Helmholtz times of several thousand years. The number of these sources imply a lifetime of ∼6000 yr, in closer agreement with the Kelvin-Helmholtz time. In this case, rotational and/or magnetic support could be slowing the collapse.
Filament Rotation in the California L1482 Cloud Álvarez-Gutiérrez, R. H.; Stutz, A. M.; Law, C. Y. ...
Astrophysical journal/The Astrophysical journal,
02/2021, Volume:
908, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
We analyze the gas mass distribution, the gas kinematics, and the young stellar objects of the California Molecular Cloud L1482 filament. The mean Gaia DR2 YSO distance is
pc. In terms of ...the gas, the line-mass (M/L) profiles are symmetric scale-free power laws consistent with cylindrical geometry. We calculate the gravitational potential and field profiles based on these. Our IRAM 30 m multi-tracer position–velocity diagrams highlight twisting and turning structures. We measure the C
18
O velocity profile perpendicular to the southern filament ridgeline. The profile is regular, confined (projected
r
≲ 0.4 pc), antisymmetric, and, to first order, linear, with a break at
r
∼ 0.25 pc. We use a simple solid-body rotation toy model to interpret it. We show that the centripetal force, compared to gravity, increases toward the break; when the ratio of forces approaches unity, the profile turns over, just before the implied filament breakup. The timescales of the inner (outer) gradients are ∼0.7 (6.0) Myr. The timescales and relative roles of gravity to rotation indicate that the structure is stable, long lived (∼a few times 6 Myr), and undergoing outside-in evolution. This filament has practically no star formation, a perpendicular Planck plane-of-the-sky magnetic field morphology, and 2D “zig-zag” morphology, which together with the rotation profile lead to the suggestion that the 3D shape is a “corkscrew” filament. These results, together with results in other regions, suggest evolution toward higher densities as rotating filaments shed angular momentum. Thus, magnetic fields may be an essential feature of high-mass (
M
∼ 10
5
M
⊙
) cloud filament evolution toward cluster formation.
Aims.
We aim to characterize the outflow properties of a sample of early Class 0 phase low-mass protostars in Orion, which were first identified by the
Herschel
Space Observatory. We also look for ...signatures of infall in key molecular lines.
Methods.
Maps of CO
J
= 3–2 and
J
= 4–3 toward 16 very young Class 0 protostars were obtained using the Atacama Pathfinder EXperiment (APEX) telescope. We searched the data for line wings indicative of outflows and calculated masses, velocities, and dynamical times for the outflows. We used additional HCO
+
, H
13
CO
+
, and NH
3
lines to look for infall signatures toward the protostars.
Results.
We estimate the outflow masses, forces, and mass-loss rates based on the CO
J
= 3–2 and
J
= 4–3 line intensities for eight sources with detected outflows. We derive upper limits for the outflow masses and forces of sources without clear outflow detections. The total outflow masses for the sources with clear outflow detections are in the range between 0.03 and 0.16
M
⊙
for CO
J
= 3–2 and between 0.02 and 0.10
M
⊙
for CO
J
= 4–3. The outflow forces are in the range between 1.57 × 10
−4
and 1.16 × 10
−3
M
⊙
km s
−1
yr
−1
for CO
J
= 3–2 and between 1.14 × 10
−4
and 6.92 × 10
−4
M
⊙
km s
−1
yr
−1
for CO
J
= 4–3. Nine protostars in our sample show asymmetric line profiles indicative of infall in HCO
+
, compared to H
13
CO
+
or NH
3
.
Conclusions.
The outflow forces of the protostars in our sample show no correlation with the bolometric luminosity, unlike those found by some earlier studies for other Class 0 protostars. The derived outflow forces for the sources with detected outflows are similar to those found for other, more evolved, Class 0 protostars, suggesting that outflows develop quickly in the Class 0 phase.
The reactor antineutrino anomaly might be explained by the oscillation of reactor antineutrinos toward a sterile neutrino of eV mass. In order to explore this hypothesis, the STEREO experiment ...measures the antineutrino energy spectrum in six different detector cells covering baselines between 9 and 11 m from the compact core of the ILL research reactor. In this Letter, results from 66 days of reactor turned on and 138 days of reactor turned off are reported. A novel method to extract the antineutrino rates has been developed based on the distribution of the pulse shape discrimination parameter. The test of a new oscillation toward a sterile neutrino is performed by comparing ratios of cells, independent of absolute normalization and of the prediction of the reactor spectrum. The results are found to be compatible with the null oscillation hypothesis and the best fit of the reactor antineutrino anomaly is excluded at 97.5% C.L.
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